add option to write logging output to a file

- '--write-log FILE' as cmdline argument
- 'output.logfile' as config file option

docs/gallery-dl.conf

@@ -127,6 +127,7 @@
     {
         "mode": "auto",
         "shorten": true,
-        "progress": true
+        "progress": true,
+        "logfile": null,
     }
 }